1. Field of the Invention
The present invention relates to a method of forming a film on an electronic part metal surface having burrs and, more particularly, to a method of forming an insulating film on the surface of an electronic part having a configuration such that a metal portion having burrs of the electronic part has slot portions in which windings are to be disposed.
2. Description of the Prior Art
Electronic parts having a structure such that windings are to be disposed in slot portions of a metal portion of the part, including, for example, a rotor core for a motor, are generally given an insulation treatment between each of the windings and the metal portion in order to avoid electrical shortcircuiting between the windings and the metal portion.
The configuration of the rotor core for motor is schematically shown, in top plan view, by way of example in FIG. 1. The rotor core shown in FIG. 1 has three slot portions, but the number of slot portions is not limited to three. Of course, rotor cores having four or more slot portions exist and there are also known rotor cores of various different configurations. Depending upon the size of the motor, though, thin rotor core sheets of the configuration shown in FIG. 1 are superposed in plural layers to form a rotor core, each rotor core sheet having, for example, a thickness of about 0.35 mm and a diameter of about 40 mm. Each slot portion 6 is provided with a conductor winding. The rotor core 2 is rotatable around a rotary shaft 1 in the direction of the arrow e.
FIG. 2 shows a schematic section taken along line d--d' with respect to a slot portion 6 of the rotor core 2 in FIG. 1. A plurality of thin rotor core sheets 2' are placed one over another with an insulating film 8 formed therearound. A winding (not shown) is placed around the insulating film. In case that a blanking burr 3 is exposed, the tip of the blanking burr 3 may damage a film for the winding, thus causing a short circuit between the winding and the rotor core 2. To avoid such shortcircuiting, the surface of the rotor core 2 is fit with fabrications as an insulating film 8 made of a thermoplastic resin, such as PBT, PET, PPS, or PA.
Such an insulating film is formed by applying a coating on the electronic part surface by powder coating, electrodeposition coating, or otherwise, followed by baking, whichever coating method may be employed, however, painting and baking, when carried out by using a conventional paint as in the past, will result in a lack of hiding 10 in FIG. 3(a) at the tip of a post-baking blanking burr 3 that is attributable to the surface tension of the coating. As a result, the tip of the blanking burr 3 is exposed, so that the required insulation between the winding and the rotor core for motor cannot be attained.
In order to prevent such blanking burr tip exposure and also to prevent a winding--rotor core shortcircuiting due to conductor biting into the insulating film due to coil winding, it is necessary to provide a comparatively thick film of the order of 100 to 200 .mu.m. Such a thick film brings about such a shortcoming that the proportion of the insulating film to the internal volume of slot portion 6 becomes high. For purposes of weight and thickness reduction and capacity increase with respect to motors, it has been desired that some good reduction is achieved in the thickness of the insulating film provided between the winding and the rotor core 2.
Further, blasting for deburring not only involves a cost disadvantage, but also it causes a change in the magnetic characteristics of the core 2 which poses a problem that there will be a 5% or more decrease in torque generation as compared to the case where blasting is not carried out.